Characterising the local and intense water cycle during a cold air outbreak in the Nordic Seas

Air masses in marine cold air outbreaks (CAOs) at high latitudes undergo a remarkable diabatic transformation because of the uptake of heat and moisture from the ocean surface, and the formation of precipitation. In this study, the fundamental characteristics of the water cycle during an intense and persistent, yet archetypal basinwide CAO from Fram Strait into the Nordic seas are analyzed with the aid of the tracer-enabled mesoscale limited-area numerical weather prediction model COSMO. A water budget of the CAO water cycle is performed based on tagged water tracers that follow moisture picked up by the CAO at various stages of its evolution. The atmospheric dynamical factors and boundary conditions that shape this budget are thereby analyzed. The water tracer analysis reveals a highly local water cycle associated with the CAO. Rapid turnover of water vapor results in an average residence time of precipitating waters of about one day. Approximately one-third of the total moisture taken up by the CAO falls as precipitation by convective overturning in the marine CAO boundary layer. Furthermore, precipitation efficiency increases as the CAO air mass matures and is exposed to warmer waters in the Norwegian Sea. These properties of the CAO water cycle are in strong contrast to situations dominated by long-range moisture transport that occur in the dynamically active regions of extratropical cyclones. It is proposed that CAOs in the confined Nordic seas provide a natural laboratory for studying local characteristics of the water cycle and evaluating its representation in models.publishedVersio

The role of land and ocean evaporation on the variability of precipitation in the Yangtze River valley

The Yangtze River valley (YRV) experiences large intraseasonal and interannual precipitation variability, which is mainly due to East Asian monsoon influence. The East Asian monsoon is caused by interaction of many processes in the coupled land–atmosphere–ocean system. To better understand YRV precipitation variability in this complex system, we have studied the precipitation moisture sources and their connection to YRV precipitation. We obtained the moisture sources by using the European Centre for Medium-Range Weather Forecasts' (ECMWF) ERA-Interim reanalysis dataset, the FLEXible PARTicle dispersion model (FLEXPART), and the WaterSip moisture source diagnostic. The variability of moisture sources reflects the variability of YRV precipitation. Intraseasonal variations of moisture sources include a shift of the most important source regions as the monsoon progresses. Interannual variability of the moisture sources shows that sources which are less important climatologically are closely connected to variations of the driest and wettest years. Our results show that land directly contributes 58 % of moisture for YRV precipitation during 1980–2016, whereas the ocean contributes 42 % in direct transport. While the importance of the ocean as a moisture source is often emphasized, our results underscore the importance of the process of continental recycling and the role of land moisture sources.publishedVersio

Synoptic Conditions and Moisture Sources Actuating Extreme Precipitation in Nepal

Despite the vast literature on heavy‐precipitation events in South Asia, synoptic conditions and moisture sources related to extreme precipitation in Nepal have not been addressed systematically. We investigate two types of synoptic conditions—low‐pressure systems and midlevel troughs—and moisture sources related to extreme precipitation events. To account for the high spatial variability in rainfall, we cluster station‐based daily precipitation measurements resulting in three well‐separated geographic regions: west, central, and east Nepal. For each region, composite analysis of extreme events shows that atmospheric circulation is directed against the Himalayas during an extreme event. The direction of the flow is regulated by midtropospheric troughs and low‐pressure systems traveling toward the respective region. Extreme precipitation events feature anomalous high abundance of total column moisture. Quantitative Lagrangian moisture source diagnostic reveals that the largest direct contribution stems from land (approximately 75%), where, in particular, over the Indo‐Gangetic Plain moisture uptake was increased. Precipitation events occurring in this region before the extreme event likely provided additional moisture.publishedVersio

High-resolution stable isotope signature of a land-falling atmospheric river in southern Norway

Heavy precipitation at the west coast of Norway is often connected to elongated meridional structures of high integrated water vapour transport known as atmospheric rivers (ARs). Here we present high-resolution measurements of stable isotopes in near-surface water vapour and precipitation during a land-falling AR in southwestern Norway on 7 December 2016. In our analysis, we aim to identify the influences of moisture source conditions, weather system characteristics, and post-condensation processes on the isotope signal in near-surface water vapour and precipitation. A total of 71 precipitation samples were collected during the 24 h sampling period, mostly taken at sampling intervals of 10–20 min. The isotope composition of near-surface vapour was continuously monitored in situ with a cavity ring-down spectrometer. Local meteorological conditions were in addition observed from a vertical pointing rain radar, a laser disdrometer, and automatic weather stations. We observe a stretched, “W”-shaped evolution of isotope composition during the event. Combining paired precipitation and vapour isotopes with meteorological observations, we define four different stages of the event. The two most depleted periods in the isotope δ values are associated with frontal transitions, namely a combination of two warm fronts that follow each other within a few hours and an upper-level cold front. The d-excess shows a single maximum and a step-wise decline in precipitation and a gradual decrease in near-surface vapour. Thereby, the isotopic evolution of the near-surface vapour closely follows that of the precipitation with a time delay of about 30 min, except for the first stage of the event. Analysis using an isotopic below-cloud exchange framework shows that the initial period of low and even negative d-excess in precipitation was caused by evaporation below cloud base. The isotope signal from the cloud level became apparent at ground level after a transition period that lasted up to several hours. Moisture source diagnostics for the periods when the cloud signal dominates show that the moisture source conditions are then partly reflected in surface precipitation and water vapour isotopes. In our study, the isotope signal in surface precipitation during the AR event reflects the combined influence of atmospheric dynamics, moisture sources, and atmospheric distillation, as well as cloud microphysics and below-cloud processes. Based on this finding, we recommend careful interpretation of results obtained from Rayleigh distillation models in such events, in particular for the interpretation of surface vapour and precipitation from stratiform clouds.publishedVersio

Linking Sub-Tropical Evaporation and Extreme Precipitation Over East Antarctica: An Atmospheric River Case Study

We investigate an intense snowfall event between 15 and 18 February 2011 over the East Antarctic coastal region which contributed to roughly 24% of the annual snow accumulation. The event was previously associated with an atmospheric river, and here we use both Eulerian and Lagrangian analysis to gain an understanding of the processes contributing to the atmospheric river signature. The planetary-scale configuration during the event consisted of a persistent blocking situation resulting in a sustained meridional flow from the sub-tropics to the Antarctic ice sheet between 20 and 50°E. Within this configuration, synoptic-scale cyclogenesis contributed to slantwise ascent of moisture loaded air parcels toward Antarctica. Landfall of this cyclone’s warm sector coincided with the onset of Antarctic precipitation. Subsequently, a secondary cyclone developed along a pre-existing baroclinic zone. The rapid intensification and propagation speed of this mesoscale cyclone alongside the warm, moist air mass resulted in strong moisture flux convergence ahead of the cyclone, providing additional poleward moisture transport. The poleward progression of warm moist air and a corresponding decrease of sea-surface temperatures implied downward surface sensible and latent heat fluxes throughout the region of intense poleward moisture, roughly between 40 and 60°S. Hence, moisture uptake via surface evaporation was suppressed between the sub-tropics and the polar continent, favoring long-range transport. Identification of the surface moisture uptake region by tracing changes in moisture in air parcels confirmed the limited uptake of moisture during the poleward transport in this case study, with the primary moisture source for Antarctic precipitation located in the sub-tropics.publishedVersio

FLIIMP - a community software for the processing, calibration, and reporting of liquid water isotope measurements on cavity-ring down spectrometers

Precise and accurate measurements of the stable isotope composition from precipitation, land ice, runoff, and oceans provide critical information on Earth's water cycle. The analysis, post-processing, and calibration of raw analytical signals from laser spectrometers during sample analysis involves a number of critical procedures to counteract instrumental drift, inter-sample memory effects, and the quantification of total uncertainty. We present a new software tool for the post-processing and calibration named FLIIMP (FARLAB Liquid Water Isotope Measurement Processor). FLIIMP facilitates sample processing by (1) a graphical user interface that guides the user along the processing steps from corrections for memory effects, drift, and mixing ratio to calibration, and (2) allows to monitor long-term measurement system behaviour, currently for Picarro-brand water isotope analysers. Final data files are accompanied by a detailed calibration report. Being an open-source software for the major operating systems, users can adapt FLIIMP to their laboratory environment, and the community can contribute the software development. • FLIIMP facilitates post-processing, calibration and reporting for stable water isotope liquid sample analysis. • The stepwise, interactive graphical user interface reduces possibility of errors and shortens processing time. • Open source software enables future development of FLIIMP by the user community.publishedVersio

Asymmetries in the moisture origin of Antarctic precipitation

The seasonality of moisture sources for precipitation in Antarctica is studied with a Lagrangian moisture source diagnostic. Moisture origin for precipitation in Antarctica has strongly asymmetric properties, which are related to the Antarctic topography, seasonal sea ice coverage, and the land/ocean contrasts in the mid-latitudes of the southern hemisphere. The highest altitudes of the East Antarctic ice shield, where major ice cores have been drilled, have mean source latitudes of 45–40°S year-round. This finding contrasts to results from previous Lagrangian studies which detected a more southerly moisture origin due to too short trajectories. Now, results from Lagrangian moisture source diagnostics are consistent with findings from general circulation models with tagged tracers. Thus, both approaches can serve as a common benchmark for the interpretation of moisture source indicators based on stable isotopes, such as deuterium excess, in Antarctic ice cores

A new interpretative framework for below-cloud effects on stable water isotopes in vapour and rain

Raindrops interact with water vapour in ambient air while sedimenting from the cloud base to the ground. They constantly exchange water molecules with the environment and, in sub-saturated air, they evaporate partially or entirely. The latter of these below-cloud processes is important for predicting the resulting surface rainfall amount. It also influences the boundary layer profiles of temperature and moisture through evaporative latent cooling and humidity changes. However, despite its importance, it is very difficult to quantify this process from observations. Stable water isotopes provide such information, as they are influenced by both rain evaporation and equilibration (i.e. the exchange of isotopes between raindrops and ambient air). This study elucidates this option by introducing a novel interpretative framework for stable water isotope measurements performed simultaneously at high temporal resolution in both near-surface vapour and rain. We refer to this viewing device as the ΔδΔd-diagram, which shows the isotopic composition (δ2H, d-excess) of equilibrium vapour from precipitation samples relative to the ambient vapour. It is shown that this diagram facilitates the diagnosis of below-cloud processes and their effects on the isotopic composition of vapour and rain since equilibration and evaporation lead to different pathways in the two-dimensional phase space of the ΔδΔd-diagram, as investigated with a series of sensitivity experiments with an idealized below-cloud interaction model. The analysis of isotope measurements for a specific cold front in central Europe shows that below-cloud processes lead to distinct and temporally variable imprints on the isotope signal in surface rain. The influence of evaporation on this signal is particularly strong during periods with a weak precipitation rate. After the frontal passage, the near-surface atmospheric layer is characterized by higher relative humidity, which leads to weaker below-cloud evaporation. Additionally, a lower melting layer after the frontal passage reduces time for exchange between vapour and rain and leads to weaker equilibration. Measurements from four cold frontal events reveal a surprisingly similar slope of ΔdΔδ=−0.30 in the phase space, indicating a potentially characteristic signature of below-cloud processes for this type of rain event.publishedVersio